Process and apparatus for purifying and separating compressed gas mixtures



23, 1954 c. MATSCH ETAL 3,119,676

PROCESS AND APPARATUS FORPURIFYING AND SEPARATING COMPRESSED GAS MIXTURES Original Filed Aug. 12, 1957 2 Sheets-Sheet 1 I I a a I m 8 7 Q 5: E INVENTORS 8 WILBUR H.LAUER N EDWARD F.YENDALL LADISLAS C.MATSCH HELMUT KOEHN A T TORNE V Jan. 28, 1964 C. MATSCH ETAL PROCESS AND APPARATUS FOR PURIFYING AND SEPARATING COMPRESSED GAS MIXTURES 2 Sheets-Sheet 2 Original Filed Aug. 12, 1957 INVENTORS WILBUR H. LAUER EDWARD F. YENDALL LADISLAS C. MATSCH HELMUT KOEHN A T TORNEY United States Patent PROCESS AND APPARATUS FOR PYING AND SEEARATHNG COMPRESSED GAS MHXTURES Ladislas C. Matsch and Edward F. Yendall, Kenmore,

Wilbur H. Lauer, Snyder, and Helmut Koehn, Tonawauda, N.Y., assignors to Union Carbide Corporation, a corporation of New York Original application Aug. 12, 1957, Ser. No. 677,606, new Patent No. 2,984,079, dated May 16, 1961. Divided and this application Oct. 11, 1960, Ser. No. 75,648

2 Claims. (Cl. 62-43) This invention relates to an improved process of and apparatus :for purifying and separating compressed gas mixtures, and more particularly to improved process and apparatus for the separation of water and carbon dioxide impurities from compressed air prior to low temperature rectification of such air into air components. This invention is a division of application Serial No. 677,606, filed August 12, 1957, now Patent No. 2,984,079.

Atmospheric air contains substantial quantities of water and carbon dioxide impurities, and unless these impurities are removed by chemical treatment of the air, or by adsorption therefrom, they will deposit as solid particles on the air side heat exchange surfaces as the air is cooled. This causes considerable difficulty, because if such deposition is continued the air side heat exchange surfaces become coated with thick layers of solid particles thus reducing heat transfer efficiency. Eventually these surfaces will plug up completely, making the air separation process inoperative. One solution to this problem is to utilize duplicate heat exchangers piped in parallel so that a clogged heat exchanger may be thawed while the other is in use. However, such duplication is an expensive solution because the heat exchangers represent a major item of air separation plant investment cost.

In air separation plants employing relatively low air 9 supply pressures, most of the water and carbon dioxide impurities are removed from the incoming air and deposited in a reversible heat exchange zone by heat exchange with outgoing air separation products. This zone may comprise heat exchangers of the regenerative or passage exchanging types. In order to avoid a build-up of ice and carbon dioxide solid particles in such heat exchange zone, the zone must be seltcleaning. This means that all of the impurities deposited in the zone during an air intake stroke must be evaporated and swept out during the next succeeding product gas stroke. The self-cleaning condition may not be achieved by simply passing all of the outgoing product gas through the reversible heat exchange zone because compressed air, especially at low temperatures, has a substantially greater specific heat than the non-compressed air separation products, e.g. oxygen and nitrogen.

The prior art has devised many ways of alleviating this condition, one of which involves partially cooling the incoming air stream in the reversible heat exchange zone and depositing the water impurity in such zone. A minor portion of the partially cooled air stream is withdrawn from the zone and separately cleaned, while the major portion of the air stream is further cooled in the reversible heat exchange zone. Most of the major portions carbon dioxide content is removed by deposition in such zone. Since by this arrangement, the volume of outgoing air separation products passing through the colder part of the zone is substantially greater than the volume 2 of incoming air passing through this part, the reversible heat exchange zone can be made self-cleaning. One

method of cleaning the diverted minor portion of partially cooled air, or side-bleed, is to chill the latter well below the carbon dioxide deposition point by direct mixing with a small part of the further cooled or cold end air having passed through the entire length of the reversible heat exchange zone. Unfortunately this scheme has several important disadvantages. If direct mixing is used, local precipitation of carbon dioxide is likely to occur at the point of mixing, thus requiring a filter to remove the solid particles. Also, if colder air is mixed with the side-bleed air, a larger quantity of air must be processed by the filter, and a larger filter must be used to avoid a higher pressure drop. For control purposes, it is desirable to maintain the pressure drop in the sidebleed circuit as low as possible. Furthermore, to mix cold end air with the side-bleed air, the latter must be slightly throttled. This is because the undiverted air is subjected to additional pressure drop in passing through the colder part of the reversible heat exchange zone. As a result of the side-bleed throttling necessity, if any part of the carbon dioxide-free throttled side bleed air is substantiallyto be bypassed to the cold end air stream, the latter must also be slightly throttled to obtain flow in the desired direction. Throttling of the cold end air is undesirable as it substantially increases the air compression power costs.

Another problem connected with the side-bleed method of unbalancing a reversible heat exchange zone for selfcleaning is determining the temperature level for such side-bleeding. The ideal temperature range for side-bleeding regenerators or reversing heat exchangers is C. to C. There are several reasons for this selection, as follows:

(1) The side-bleed air volume necessary for self-cleaning is less at warmer levels than at colder levels. For example, the required volume at the C. level is 30% to 40% greater than at the --l0(l C. level.

(2) A warmer side-bleed point avoids approaching too closely the level at which carbon dioxide begins to deposit, e.g. approximately 134 C. for 75 p.s.i. air. The safe margin for trouble-free operation of regeneartors or reversing heat exchangers is substantially reduced by lowering the side-bleed level below --120 C.

(3) In the case of reversing heat exchangers, the units are normally available in standard lengths, and this conveniently fixes the temperature between the Warm and cold units at approximately -l00 C. Withdrawing the side-bleed at a colder level would require remanifolding the complex passages at an intermediate point in the cold unit. Although this can be done, it adds appreciably to the cost of the exchangers.

One practical method of removing the carbon dioxide impurity from the side-bleed is by gas phase adsorption using an adsorbent such as silica gel. In this manner the dissolved carbon dioxide may be removed while still in solution, thereby avoiding the diifioulties of solid precipitation. Unfortunately, the ideal temperature for side-bleeding the reversible heat exchange zone does not correspond with the ideal operating temperature range for a silica gel trap, the latter being 120 C. to ---l30 C. This range is advantageous because the carbon dioxide adsorbing capacity of silica gel is considerably higher at lower temperatures.

A furtherproblem arises if an air stream is to be expandedwith the production of external work and lowtemperature refrigeration. The side-bleed air is a convenient source of warm gas for such work expansion, but either or both its temperature and volume may be unsuitable for such work expansion. For example, the optimum expansion turbine inlet temperature is approximately 153 C., which of course is substantially below either the optimum side-bleed or silica gel adsorption temperature. The turbine inlet temperature is selected so as to operate as cold as possible and yet avoid appreciable liquid condensation in the turbine discharge. The latter is undesirable as it produces erosion of the turbine blades and loss of efliciency. Also, the addition of colder air to the side-bleed air to obtainthis optimum temperature level for the turbine will generally resultin alarger' volume ofair than is desired for work expansion. The quantity of air to be workexpandedis determined by a heat balance on the air separation cycle. It is undesirable to Work expand more air than is required by the heat balance, because such excess air expansion would reduce the efliciency of the process and may require an oversized turbine.

Principal objects of the present invention are to provide a process and apparatus for purifying and separating compressed air utilizing a side-bleed for unbalancing the reversible heat exchange zone, means for removing the carbon dioxide impurity from the side-bleed, and means for Work expanding at least part of the sidebleed, the steps and apparatus being-arranged and related so that each step is conducted under ideal or optimum conditions, the overall result being a highly eflicient and low cost system for separation of air into products or components.

These and other objects and'advantages of this invention will be apparent from the following description and accompanying drawings in which:

FIG. 1 shows a fiow diagram of a system for purifying an air'stream and preparing a portion of such stream for work expansion, according to the present invention;

FIG. 2 is a flow diagram of still another system but modified so that a scrubbed clean air stream from the rectification column is work expanded along with part of the air side-bleed.

According to the present invention, an air stream at an inlet pressure below 150 p.s.i is passed to a reversible heat exchange zone, partially cooled to at least -80 C. for removal of the water impurity by deposition in such Zone, and divided into major and minor portions. The minor or side-bleed portion is withdrawn from the zone and further cooled by heat exchange with a first colder fluid to a temperature slightly above the deposition point of carbon dioxide at the inlet pressure. The dissolved carbon dioxide impurity is then removed by adsorption from this-further cooled side-bleed stream. Also, the major portion of the air stream is furthercooled in the reversible heat exchange zone, and at least most of the carbon dioxide impurity of such portion is removed by deposition in the colder part of such zone; At least part of the further cooled carbon dioxide-free major portion, or cold end air, is passed to a rectification zone for separation into air components. The water and carbon dioxide impurities deposited in the reversible heat exchange zone are removed by passing at least part of the separated air components through such zone to evaporate such impuritiestherein for discharge from the zone.'

In one embodiment of'the invention; a minor part of the cold end air is diverted and passed in cocurrent heat exchangewith the partiallycooled side-bleed air as the first colder fluid.

If an air streamis to be Work expanded, this invention provides a method of forming an expander 'inle't streamhaving suitable flow and temperature'so as to achieve optimum work expansion conditions as Well as self-cleaning conditions. This may'be accomplished by diverting a regulated part of the partially cooled carbon dioxidefree side-bleed air, slightly throttling the undiverted sidebleed, and diverting a regulated part of the cold end air to the slightly throttled undiverted side-bleed, thus pro- Viding the desired expander inlet stream.

It can be seen from the previous brief descriptions that this invention permits ideal operating conditions in the various steps of the air separation cycle, the overall result being a highly efiicient and economical method of obtaining air separation products such as oxygen and nitrogen. Furthermore, this invention eliminates the previously described disadvantages of the direct mixing and chilling method of treating the air side-bleed. In the present invention, there is no direct mixing with cold end air prior to carbon dioxide removal, hence the side-bleed is cooled to the ideal temperature for carbon dioxide removal without the necessity of using a filter or a larger silica gel trap. Furthermore, it will be noted that no throttling of the side-bleed occurs prior to carbon dioxide removal, hence cleaned side-bleed air may be diverted to the cold end air without having to throttle the latter stream to induce flow in the desired direction.

Referring now to the drawings and particularly to FIG. 1, air is compressed in compressor 10 to a pressure of less than 150 p.s.i.g. and preferably about p.s.i.g., and the heat of compression may be removed by, for example, a water-cooled exchanger (not shown). The compressor discharge air stream passes through conduit 11 and reversing valves 12 to a reversible heat exchange zone 13, which may comprise regenerators or reversing heat exchangers. In zone 13 the air stream is cooled by heat exchange with air separation products such as oxygen or nitrogen, such products entering the cold end of the reversing heat exchange zone 13 through conduit 14 and reversing valves 14a therein, and emerging through conduit 15 at the warm end of zone 13. The manner of cooling the air stream by heat exchange either with a colder fluid in an adjacent passageway, orthrough. an intermediate refrigeration storage means such as regenerative packing, is Well-known to those skilled in the art and described in Fr'ainkl U.S.P. 1,890,646 for regenerators, and Trumpler U.S.P. 2,460,859 exchangers. Reversing valves 12 and 14a are suitably connected to each other and to the zone 13 in order to achieve this cyclic heat exchange.

The water content of the inlet air stream is removed by deposition in the warmer part of the reversible heat exchange'zone 13, and such stream is divided into major and minor portions by withdrawing a minor portion or sidebleed at approximately C. through conduit 16 and valve 17 therein. The side-bleed air may constitute approximately 3% to 15% of the total inlet air stream, and preferably about 10%. One purpose of the side-bleed is to. bypass a sufficient portion of the inlet air stream around the colder part of the reversing heat exchange zone 13 so that the flow ratio of outgoing air separation products to inlet air will be suificiently increased to achieve a selfcleaning temperature pattern inthe zone 13. As previously discussed, it 'may be preferable'to withdraw the sidebleed air at the 80 C. to -l00 C. level instead of a lower temperature level to minimize the side-bleed flow and positively avoid carbon dioxide deposition in the heat exchange zone 13 above the side-bleed level.

The partially cooled minor air portion or side-bleed is conducted through conduit 16 to passageway 18 where it engages in cocurrent heat exchange (i.e. in the same direction) with a first colder fluid in passageway 19, and is itself further cooled to approximately C. This is an optimum temperature for silica gel adsorption of carbon dioxide from 75 p.s.i. air which still provides a safe margin above the deposition point of carbon dioxide at this pressure. At these conditions, carbon dioxide begins depositing at approximately 134 C., and if the side-bleed air were cooled appreciably below about -1 20 C., it is probable that some carbon dioxide deposition for reversing heat would occur in the heat exchange zone 13 above the sidebleed level, in passageway 18, and in the interconnecting piping, and this could eventually necessitate a shut-down for thaw. The further cooled side-bleed is passed from passageway 18 through conduit 19a and inlet valves 20 into one or the other of a pair of the silica gel traps 21 for removal of the still dissolved carbon dioxide by silica gel adsorption. These gel traps are provided in duplicate and piped in parallel for alternate operation so that when one gel trap becomes loaded with carbon dioxide, the side bleed air may be diverted to the other gel trap having previously been purged and reactivated by means not illustrated. As previously discussed, it is preferable to conduct the adsorption step at relatively low temperatures, such as 120 C., because the carbon dioxide adsorptive capacity of silica gel is higher at lower temperatures. The carbon dioxide-free side-bleed air emerges through silica gel trap discharge valves 22 into conduit 23.

Meanwhile, the major portion of the inlet air stream is further cooled in the reversible heat exchange zone 13, and most of its carbon dioxide impurity is removed by deposition in the colder part of such zone. The further cooled major portion is discharged from the cold end of zone 13 at approximately 173 C. into conduit 24, and thence passes to cold end trap 25 for silica gel filtration and adsorption of any residual carbon dioxide not previously removed. The carbon dioxide-free further cooled air, or cold end air, is discharged from silica gel trap 25' into conduit 26.

In this particular cycle, it is desirable to expand an air stream with the production of external work, but the carbon dioxide-free side-bleed air stream is too warm (l20 C.) and may be too large or too small in volume for optimum work expansion conditions. An air stream having suitable flow and temperature for work expansion may be formed by first diverting a regulated part e.g. 60%, of the side-bleed through conduit 27 and regulating valve 28, and still further cooling such diverted part to approximately 170 C. by passage through passageway 29 in heat exchange with a colder fluid flowing through passageway 31). Such colder fluid may be, for example, nitrogen product from the rectification column. This diverted stream still further cooled in conduit 27 is then mixed with the cold end air either downstream or upstream of the cold end silica gel trap 25. The further cooled major portion or cold end air is passed through conduit 26 to the rectification column (not shown) for separation into products such as oxygen and nitrogen in a manner wellknown to those skilled in the art. Alternatively, the diverted part in conduit 27 may be passed directly to the rectification column instead of united with cold end air.

In the previously described system, it is to be noted that air flow is obtained in the desired direction, that is, from the side-bleed stream to the cold end stream, because the pressure drop in the side-bleed circuit is less than the pressure drop through the colder part of the reversing heat exchange zone 13 and the cold end gel trap 25. Since cold end air is to be diverted from conduit 26 to the carbon dioxide-free side-bleed in conduit 23 to cool the latter stream, it is necessary to slightly throttle the side-bleed by means of valve 31 to a pressure just below the pressure of the cold end air in conduit 26. A regulated part, e.g. 18% of the latter, is diverted through conduit 32 and passed through passageway 19 in cocurrent heat exchange with the side-bleed in conduit 18 to further cool the latter as said first colder fluid. A bypass conduit 33 and valve 34 are provided to insure flexibility of operation.

Cocurrent heat exchange assures that the metal wall temperature in the heat exchanger formed by passageways 18 and 19 will not drop below 134 C. at any point, and hence carbon dioxide will not be deposited therein. Cocurrent flow, although less efiicient than countercurrent flow, is preferred when cold end air is used for cooling the side-bleed air, since this arrangement places the warmest portion of the side-bleed air (at C.) adjacent to the coldest portion of the cold end air (at -173 C.). Thus, the arithmetic average of the temperatures at the inlet end of the cocurrent heat exchanging passageways 18 and 19 is about -136 C., and the metal wall temperature can easily be raised Well above the carbon dioxide deposition range by increasing the heat transfer surface on the air side-bleed passageway 18, which is the warm side of the heat exchanger. In practice, it is preferable to provide a warm side hA value (heat transfer coefficient area) about 25% greater than the hA value for the cold side.

It is possible to employ countercurrent heat exchange, but in this case the coldest portion of the side-bleed air (at C.) is adjacent to the coldest portion of the cold end air at 173 C., and the arithmetic average is C. Thus, it is more difficult to design and operate a countercurrent heat exchanger to maintain the walls of passageway 18 above the critical -134 C. temperature.

The partially rewarmed diverted cold end air in conduit 32 mixes with the slightly throttled side-bleed from conduit 23 to form a turbine inlet stream in conduit 35. The flow of this stream and the diverted cold end air in conduit 32 are regulated by valve 36 in conduit 35. In this manner, an expander inlet stream is formed so as to achieve optimum work expansion conditions, and the regulated inlet stream in conduit 35 at about 153 C. enters turbine 37 for work expansion therein to about 3 psi. and -183 C. The turbine discharge stream in conduit 38 may either be passed to the rectification column for separation into air components, or united with the portion of these components passing to the reversible heat exchange zone 13 for cooling and partially cleaning the inlet air therein. A further possibility is dividing the work expanded side-bleed air so that a part thereof may be directed to each of the aforementioned points. In any event, this stream contains a substantial quantity of refrigeration, and the cycle efiiciency is greatly improved when such refrigeration is recovered.

FIG. 2 illustrates still another embodiment of the invention in which the cold end air and a diverted part of the cleaned side-bleed stream pass to a scrubbing section in the base of the rectification column for removal of residual carbon dioxide impurity. A regulated portion of the scrubbed air is then withdrawn, preheated, mixed with the undiverted cleaned side-bleed to form an expander inlet stream having optimum conditions, and work expanded in the previously described manner. The advantage of this embodiment is elimination of the cold end adsorption trap.

Referring now to FIG. 2 in detail, the side-bleed stream 316 is further cooled in passageway 318 by cocurrent heat exchange with a first colder fiuid in passageway 319 and cleaned in adsorbers 321. The cleaned side-bleed stream is discharged from such adsorbers into conduit 323, and a regulated part of such stream is diverted through conduit 327 and regulating valve 328 therein to passageway 350, where it is further cooled by heat exchange with the first colder fluid in passageway 351. The further cooled diverted side-bleed part is combined with cold end air in conduit 324, and the combined stream is directed to a scrubbing section 342 in the base of the higher pressure chamber 343 of the rectifying device 344. The scrubbing section 342 contains liquid air which thoroughly scrubs the incoming air and retains the residual carbon dioxide in the scrubber liquid. Such scrubber liquid is withdrawn through a conduit 345 for impurity removal in devices represented by filters 346 after passage through filter inlet valves 347. The cleaned scrubber liquid is discharged from the filters through discharge valves 348 into conduit 349, and passed through throttling valve 354 into the lower pressure chamber 355 of the rectifying device 344. This liquid enters the chamber 355 at an intermediate level and serves as reflux liquid.

The rectifying device 344 includes the higher pressure chamber 343'which, if desired, may be a rectifying column having customary gas and liquid contact devices such as trays 356 therein. The chamber 343 is closed at the top by a main condenser 357 which condenses nitrogenrich vapor at the top. Part of such condensate collects on a shelf 358 at the upper end of the chamber 343 and is transferred by a conduit 359 through a heat exchange passageway 360 therein to the upper end of the main or lower pressure chamber 355. This chamber is also provided with trays 356 and communicates at its lower end with a chamber 361 surrounding the main condenser 357. The liquid nitrogen transfer conduit 359 which has a throttling valve 362 therein connects with the top of the column 355, and the shelf transfer liquid serves as reflux liquid for such column. The effluent nitrogen leaves the upper end of chamber 355 through a conduit 363 and is conducted to a passageway 364 where it subcools the shelf transfer liquid in passageway. 360, and in turn is superheated. The partially warmed efi iuent nitrogen is then conducted by conduit 314 to the cold end of the reversible heat exchange zone 313, from the warm end of which'a conduit 315 discharges the product nitrogen. Meanwhile gaseous oxygen is withdrawn from the base of the lower pressure chamber 355 through conduit 365 and regulating valve 366 to the cold end of reversible heat exchange zone 313 for passage therethrough and discharge through conduit 367 at the warm end of such zone. It is to be noted that the oxygen product stream in conduit 365 may be recovered from zone 313 without contamination by air impurities by using either embedded coils in regenerators or a non-reversing pass in reversing heat exchangers.

Returning now to the scrubbing section 342 of the higher pressure chamber 343, a portion of the scrubbed air is withdrawn through conduit 368 and divided into two parts. One regulated part passes through conduit 369 and regulating valve 370 therein, while the other part is conducted through conduit 371 to passageway 351 where it is partially warmed and serves as the first colder fiuid which further cools the diverted cleaned side-bleed stream in passageway 350. The partially warmed scrubbed air isthen directed to passageway 319 where it is further warmed by cocurrent heat exchange with side-bleed air in conduit 318. Countercurrent heat exchange could also be used at this point with proper regulation since the scrubbed air is sufficiently warmed in passageway 351 to avoid'cooling the side-bleed air below the carbon dioxide deposition point. The further warmed scrubbed air emerging from passageway 319 into conduit 371 is mixed with the regulated part of scrubbed air in conduit 369, and the combined stream in passageway 372 is conducted to a juncture with the undiverted cleaned side-bleed stream in conduit 323 to form the turbine inlet stream in conduit 335. The flow of this stream is controlled by valve 336, and such stream is expanded in turbine 337 to a relatively low pressure, for example, 3 p.s.i.g., with the production of external work. The expanded low pressure air is discharged through conduit 338 into the lower pressure chamber 355 for rectification into air components. It can thus be seen that the FIG. 2 embodiment of the invention provides a method of forming an optimum work expander inlet stream by adjusting the volume and tempe'rature of the side-bleed and scrubbed air streams so that portions thereof may be combined to form the expander stream.

Although preferred embodiments of the invention have been described'in detail, it is contemplated that modifications of the process and the apparatus may be made and that some features may be employed without others, all within the spirit thereof as set forth in the claims. For example, instead of incorporating the scrubber in the lower column as in FIG. 4, a separate scrubbing vessel may be used in a manner well-known to those skilled in the art. The principles of the invention may also be applied to the purification and separation of other lowboiling gas mixtures containing water and carbon impurities.

What is claimed is:

1. A process for the low-temperature rectification of water and carbon dioxide impurity-containing air including the steps of providing an air stream at an inlet pressure below 150 p.s.i.; passing such stream to a reversible heat exchange zone; partially cooling the air stream inlsuch zone to at least C. and removing the water impurity by deposition in the heat exchange zone; dividing the partially cooled air'strearn into major and minor portions; withdrawing the minor portion from said reversible heat exchange zone and further cooling such portion by. heat exchange with a first colder fluid to a temperature slightly above the deposition point of carbon dioxide at said inlet pressure; removing the dissolved carbon dioxide impurity from the further cooled minor portion of said air stream by adsorption; diverting and still further cooling a regulated part of such portion by heat exchange with said first colder fluid; removing at least most of the carbon dioxide impurity from the partially cooled major portion of said air stream by further cooling and deposition thereof in said reversible heat exchange zone; passing at least part of the further cooled major portion and the still further cooled regulated part of said minor portion of the air stream to a rectification zone for separation into air components; removing the water and carbon dioxide impurities deposited in said heat exchange zone by passing at least part of the separated air components through the zone and evaporating such impurities therein for discharge from such zone; withdrawing a carbon dioxide-free gas portion from said rectification zone and dividing such portion into first and second parts; preheating said first part of such withdrawn gas by consecutive heat exchange with the diverted part of the minor portion of said air stream and the withdrawn minor portion respectively as said first colder fluid; mixing the preheated first part and the second part of the withdrawn gas portion; combining such mixed stream with the undiverted part of the further cooled carbon dioxide-free minor portion of said air stream to form a work expansion inlet stream; and expanding such stream to a lower pressure with the production of external work.

2. Apparatus for the separation of water and carbon dioxide impurity-containing air by low-temperature rectification including a rectifying device; means by which inlet air is supplied at a pressure below p.s.i.; a reversible heat exchange zone for partially cooling the inlet air to at least -80 C. so that the water impurity is deposited in such zone; means for dividing the partially cooled inlet air stream into major and minor portions; means for withdrawing the minor portion from said reversible heat exchange zone; means for further cooling the withdrawn minor portion by heat exchange with a first colder fluid to a temperature slightly above the deposition point of carbon dioxide at said inlet pressure; means comprising an adsorption trap for removing the dissolved carbon dioxide impurity from the further cooled minor portion; means for diverting and still further cooling a regulated part of the further cooled minor portion of the air stream by heat exchange passage with said first colder fluid; means for passing the still further cooled portion to said rectifying device; means for withdrawing a carbon dioxide-free gas portion from said rectifying device and means for dividing such portion into first and second parts; means for passing said first part in consecutive heat exchange with said regulated part of the further cooled minor portion and the withdrawn minor portion of the air stream respectively as said first colder fluid thereby simultaneously preheating said first part; means for mixing the preheated first part and said second part of the withdrawn minor portion; means for combining such mxed stream with the undiverted part of said further cooled minor portion to form a work expander inlet stream; means comprising a Work expander for expanding such inlet stream to a low pressure with the production of external work; means for further cooling the major portion of said air stream in said reversible heat exchange zone so that at least most of the carbon dioxide impurity is deposited in such zone; means for passing at least part of the further cooled major portion to said rectifying device for separation into air components; and means for passing at least part of the separated air components through the reversible heat exchange zone to evaporate and discharge the previously deposited air impurities from such Zone.

References Cited in the file of this patent UNITED STATES PATENTS Schuftan Ian. 27, 1942 Rice Oct. 20, 195-3 Yendall Feb. 25, 1958 Linde Mar. 4, 1958 Linde Mar. 25, 1958 Karwat Mar. 25, 9523 Wucherer July 21, 1959 Simonet Oct. 4, 1960 FOREIGN PATENTS France Feb. 15, 1932 

2. A PROCESS FOR THE LOW-TEMPERATURE RECTIFICATION OF WATER AND CARBON DIOXIDE IMPURITY-CONTAINING AIR INCLUDING THE STEPS OF PRIOVIDING AN AIR STREAM AT AN INLET PRESSURE BELOW 150 P.S.I., PASSING SUCH STREAM TO A REVERSIBLE HEAT EXCHANGE ZONE, PARTIALLY COOLING THE AIR STREAM IN SUCH ZONE TO AT LEAST -80*C. AND REMOVING THE WATER IMPURITY BY DEPOSITION IN HEAT EXCHANGE ZONE, DIVIDING PARTIALLY COOLED AIR STREAM INTO MAJOR AND MINOR PORTIONS, WITHDRAWING THE MINOR PORTION FROM SAID REVERSIBLE HEAT EXCHANGE ZONE AND FURTHER COOLING SUCH PORTION BY HEAT EXCHANGE WITH A FIRST COLDER FLUID TO A TEMPERATURE SLIGHTLY ABOVE THE DEPOSITION POINT OF CARBON DIOXIDE SAID INLET PRESSURE, REMOVING THE DISSOLVED CARBON DIOXIDE IMPURITY FROM THE FURTHER COOLED MINOR PORTION OF SAID AIR STREAM NY ADSORPTION, DIVERTING AND STILL FURTHER COOLING A REGULATED PART OF SUCH PORTION BY HEAT EXCHANGE WITH SAID FIRST COLDER FLUID, REMOVING AT LEAST MOST OF THE CARBON DIOXIDE IMPURITY FROM THE PARTIALLY COOLED MAJOR PORTION OF SAID AIR STREAM BY FURTHER COOLING AND DEPOSITION THEREOF IN SAID REVERSIBLE HEAT EXCHANGE ZONE, PASSING AT LEAST PART OF THE FURTHER COOLED MAJOR PORTION AND THE STILL FURTHER COOLED REGULATED PART OF SAID MINOR PORTION OF THE AIR STREAM TO A RECTIFICATION ZONE FOR SEPARATION INTO AIR COMPONENTS, REMOVING THE WATER AND CARBON DIOXIDE IMPURITIES DEPOSITED IN SAID HEAT EXCHANGE ZONE BY PASSING AT LEAST PART OF THE SEPARATED AIR COMPONENTS THROUGH THE ZONE AND EVAPORATING SUCH IMPURITIES THEREIN FOR DISCHARGE FROM SUCH ZONE, WITHDRAWING A CARBON DIOXIDE-FREER GAS PORTION FROM SAID RECTIFICATION ZONE AND DIVIDING SUCH PORTION INTO FIRST AND SECOND PARTS, PREHEATING SAID FIRST PART OF SUCH WITHDRAWN GAS BY CONSECUTIVE HEAT EXCHANGE WITH THE DIVERTED PART OF THE MINOR PORTION OF SAID AIR STREAM AND THE WITHDRAWIN MINOR PORTION RESPECTIVELY AS SAID FIRST COLDER FLUID, MIXING THE PREHEATED FIRST PART AND THE SECOND PART OF THE WITHDRAWIN GAS PORTION, COMBINING SUCH MIXED STREAM WITH THE UNDIVERTED PART OF THE FURTHER COOLED CARBON DIOXIDE-FREE MINOR PORTION OF SAID AIR STREAM TO FROM A WORK EXPANSION INLET STREAM, AND EXPANDING SUCH STREAM TO LOWER PRESSURE WITH THE PRODUCTION OF EXTERNAL WORK. 